单例模式

public class Singleton1 {//饿汉式
    private Singleton1(){};
    static Singleton1 singleton1 = new Singleton1();
    public static Singleton1 getSingleton1(){
        return singleton1;
    }
}

public class Singleton2 {//懒汉式
    private Singleton2(){};
    static  Singleton2 singleton2 = null;

    public static  Singleton2 getSingleton2(){
        if (singleton2 == null){//如果不为null，不进入同步，加快效率
		
            //如果为null，会有多个线程进入
            synchronized(Singleton2.class){//多个线程进入此处，只有一个线程拿锁
                if (singleton2 ==null){//只有一个线程new实例，其他线程跳出
                    singleton2 =new Singleton2();
                }
            }
			
        }
        return singleton2;
    }
}